Which pair of atoms would form a covalent bond ?
calcium (ca) and bromine (br)
rubidium (rb) and sulfur (s)
cesium (cs) and nitrogen (n)
oxygen (o) and chlorine (cl)

oxygen (O) and chlorine (Cl)

As the electronegativity of an element increases It is more likely to make an ionic bond. e.g. the metallic elements. On the moderate values of electronegativity, elements tend to make covalent bond.

Oxygen and chlorine will make covalent bond because the electronegativity difference between these two elements is 0,5 so there bond will be covalent in nature.

b. examine and describe cells